1. Field of the Invention
The present invention relates to purification and polymerization of monovinyl aromatic compounds. In another aspect, the present invention relates to purification and polymerization of styrene monomers. In even another aspect, the present invention relates to apparatus, systems and processes for removing contaminants from styrene monomer feedstock. In yet another aspect, the present invention relates to apparatus, systems and processes for reducing the amount of phenylacetylene in a styrene monomer feed, wherein the apparatus, systems and processes comprise a catalyst generally having less than 0.3 weight percent palladium.
2. Description of the Related Art
In the manufacture of monovinyl aromatic polymer compounds and more particularly in the manufacture of polystyrene, a first step comprises the reaction of benzene together with ethylene to form ethylbenzene. Ethylbenzene is dehydrogenated in an EB Dehydro unit to form styrene monomers. The resulting styrene monomers are then polymerized, usually in the presence of a polymerization initiator or catalyst, to form polystyrene resin.
If the ethylbenzene is dehydrogenated one step too far, an undesirable side product, phenylacetylene, is formed. As a result, the product stream from the Dehydro unit contains styrene, ethylbenzene, and trace amounts of phenylacetylene. The ethylbenzene can be easily removed by conventional processes, such as distillation, leaving styrene monomer and phenylacetylene. However, removal of phenylacetylene is much more difficult and distillation does not suffice.
The presence of phenylacetylene in the styrene monomer feedstock has undesirable consequences on the polymerization process. In a free-radical polymerization process, the presence of phenylacetylene has detrimental effects on chain length and polymerization rate because it is a poor chain transfer agent. In an anionic polymerization process, phenylacetylene consumes a stoichiometric amount of the catalyst, such as, for example, butyllithium, wherein one molecule of butyllithium is removed from the polymerization process by each molecule of phenylacetylene. This loss of catalyst can lead not only to high costs, but also to difficulty in controlling the molecular weight of the polymerized product, an increase in the concentration of low molecular weight polymer, and the presence of unreacted styrene in the polystyrene. Residual styrene monomer, which is a suspected carcinogen, contributes to off-taste, odor, off-color and other degradation of the polystyrene.
Clearly, the presence of phenylacetylene in a styrene monomer feedstock has adverse effects on cost of polymerization, control of the polymerization process, and the quality of the resulting polystyrene.
Catalytic attempts to decrease the level of phenylacetylene in styrene monomer streams have involved the injection of high levels of hydrogen gas into the stream. The phenylacetylene is then reduced to styrene. Unfortunately, any hydrogen present in stoichiometric excess of the phenylacetylene also results in a significant conversion of styrene back to ethylbenzene, thus causing a lower styrene concentration and a lower conversion rate.
U.S. Pat. No. 5,156,816, issued to Butler et al., discloses a system for purifying styrene monomer feedstock using ethylbenzene dehydrogenation waste gas. The system comprises a palladium catalyst on a theta-alumina carrier wherein the catalyst contains 0.3 weight percent palladium.
In spite of advancements in the art, many PAR systems and methods are inefficient and suffer from catalyst failure due to plugging or attrition of the catalyst. Methods and systems for purifying monovinyl aromatic feedstock that do not suffer from the limitations of the prior art have not been described.
Thus, there is a need in the art for methods of efficiently removing phenylacetylene contaminant from aromatic polyvinyl feedstock, said methods utilizing a catalyst that does not fail due to attrition, fluidization, or other loss of activity.
There is another need in the art for a system of efficient phenylacetylene removal from aromatic polyvinyl feedstock.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.
It is an object of the present invention to provide methods of efficiently removing a phenylacetylene contaminant from aromatic polyvinyl feedstock wherein the methods comprise a catalyst that does not fail due to attrition or fluidization or other loss of activity.
It is another object of the present invention to provide a system useful for efficient removal of phenylacetylene from aromatic polyvinyl feedstock.
Thus, one embodiment of the present invention is directed to methods of purifying crude styrene monomer feedstock. The methods comprise use of a low palladium catalyst having less than 0.3 weight percent palladium.
Another embodiment of the present invention is directed to a system of purifying a crude styrene monomer feedstock. The systems of the invention comprise reducing the phenylacetylene levels of monovinyl aromatic monomer feedstock in polymerization systems by the use of either a two-bed reactor, or a pair of catalyst reactors. Each bed or reactor has injection means for injecting a phenylacetylene reducing agent, such as hydrogen gas, into the monomer reaction stream to reduce phenylacetylene into styrene. Generally the catalyst reactors comprise a low palladium catalyst having less than 0.3 weight percent palladium, preferably less than about 0.1 weight percent palladium.